Load control devices and systems control the amount of power delivered from an alternating-current (AC) power source to an electrical load, such as a lighting load, for example. Such lighting control systems typically employ a controllably conductive device, such as a thyristor or triac for example, for controlling the intensity of the lighting load. The controllably conductive device is rendered conductive at a phase angle during each half-cycle of the AC power source in response to a trigger signal received at a control input. This establishes, within each half-cycle, a conduction period where power is being delivered to the load and a non-conduction period where power is not being delivered to the load.
In a typical forward phase-control system, generation of the trigger signal is synchronized with the AC line voltage. At some time after a zero-crossing of the AC line voltage is detected, the trigger signal is generated, and the controllably conductive device is rendered conductive. The controllable conductive device remains conductive for the remainder of the AC half cycle. During the time interval between the detection of the zero-crossing and the generation of the trigger signal, the controllable conductive device is non-conductive. This time interval may also be referred to as the phase or firing angle of the system. By varying this time interval, the effective power delivered to the load is varied. Typically, this time interval is altered in response to adjustment of a dimming knob or slider by a user and/or in response to changes in a dimming signal level.
FIGS. 1A-1D depict example AC voltage waveforms as measured across the controllably conductive device. When the controllably conductive device is non-conductive, the complete AC voltage waveform, as shown in FIG. 1A, is developed across the device. At a relatively low light level, as shown in FIG. 1B, the controllably conductive device is non-conductive for a first duration 102 of the half-cycle (i.e., from the zero-crossing of the current half-cycle of the AC voltage waveform to a point within the half-cycle). The trigger signal is generated (shown as point “A”). Then, the controllably conductive device is conductive for a second duration 104 of the half-cycle. FIGS. 1C and 1D illustrate waveforms at a 50% dimming level and a relatively high light level, respectively.
At low levels of delivered power, like that depicted in FIG. 1B, even a small variation in the phase angle (and thus the conduction period) usually represents a relatively large variation in the percentage of the total RMS power delivered to the load. At these low power levels, any variation of the phase angle, whether between AC cycles or over periods of time, can be manifested as annoying and unacceptable intensity changes, including visible flickering of the light source. Since the phase angle is dependent on the detection of the zero-crossing, it is crucial that zero-cross detection be accurate and reliable. AC line conditions, however, are rarely ideal. And, less than ideal conditions can cause inaccuracy in the detection of zero-crossings, with consequent intensity variations and/or flickering, as well as other problems, especially at low levels of delivered power. One condition that can cause intensity variations and/or flickering is intermittent and/or periodic electrical noise on the AC line.
FIGS. 2A-2C illustrate example AC voltage waveforms having noise. For example, as illustrated in FIG. 2A, voltage spikes can be imposed on an AC line, which may occur when heavy equipment, such as large motor loads, are switched on and off. Electrical noise on an AC line, such as these spikes, may be incorrectly interpreted by dimming circuitry as one or more zero-crossings of the AC line voltage. Such false interpretations can lead to erratic intensity variations and/or flickering in the lighting load. Another common characteristic of electrical noise on an AC line may include a bumpy or wavy distortion, as shown in FIG. 2B, which can also cause false zero-crossing detection. The presence of harmonics of the AC fundamental on the AC line is another condition that can cause false zero-crossing detection. The presence of harmonics may change the shape of the AC line voltage waveform from a pure sinusoid to a generally sinusoidal waveform, having flattened peaks rather than round peaks, as illustrated in FIG. 2C.
One approach to mitigate the effects of noise on an AC line includes filtering the AC line voltage prior to performing zero-crossing detection. For example, the Real-Time Illumination Stability System (RTISS) uses a filter to improve the performance of a dimming system. The RTISS technology is described in commonly-assigned U.S. Pat. No. 6,091,205, issued Jul. 18, 2000, and U.S. Pat. No. 6,380,692, issued Apr. 30, 2002, both entitled Phase controlled dimming system with active filter for preventing flickering and undesired intensity changes, the entire disclosures of which are hereby incorporated by reference.
Both three-wire dimming systems and two-wire dimming systems may employ the RTISS technology. FIG. 3A depicts an example three-wire dimming system 300. FIG. 3B depicts an example two-wire dimming system 302. Both dimming systems have dimmer switches 304, 306 electrically coupled between an AC power source 308 and an electrical load 310. The dimmer switches 304, 306 are connected to the AC power source 308 by a first wire 312 (also referred to as a “hot” wire) and to the load 310 by a second wire 314 (also referred to as a “dimmed-hot” wire). However, the three-wire dimmer switch 304 also has a third wire 316 (also referred to as a “neutral” wire), which provides a path back to the return side of the AC power source 308. The two-wire dimmer switch 306 is not connected to the neutral wire 316.
The three-wire dimmer switch 304 has two waveforms available to it. A full (i.e., not switched) AC line voltage waveform 318 is available to the three-wire dimmer 304, by virtue of its third wire 316. A dimmer-voltage waveform 320, measured from the first wire 312 and the second wire 314, is also available to the three-wire dimmer 304. The three-wire dimmer switch 304 is able to use the full AC line voltage waveform 318 for filtering to determine the zero-crossings of the AC line voltage waveform of the AC power source 308 and to generate an AC load voltage waveform 322 (e.g., a dimmed-hot voltage that is measured from the second wire 314 and the third wire 316). The two-wire dimmer switch 306, on the other hand, without a path back to return side of the AC power source 308, only has the dimmer-voltage waveform 320 at its disposal, and not the full AC line voltage waveform 318.
In two-wire dimming systems, variations in phase delay associated with filtering (e.g., from the input to output of the filter) may affect the stability of the dimming system and/or the amount of error in the zero-crossing detection. Having only the dimmer-voltage waveform 320 available for filtering to determine the zero-crossings of the AC line voltage, the two-wire dimmer switch 306 may experience substantial variation in phase delay through the filter as a function of the firing angle of the controllably conductive device. To illustrate, FIG. 4 provides a plot 402 that shows how phase delay through a low-pass filter may vary as a function of firing angle in a two-wire dimmer switch. For example, as shown, a relatively large firing angle (i.e., a relatively small conduction period) may correspond to a relatively large phase delay. As the firing angle decreases (for example, from approximately 7 milliseconds to 2 milliseconds, as shown) and the conduction period increases, the phase delay decreases substantially (for example, from approximately 5.5 milliseconds to 3 milliseconds).
The variation in phase delay may affect system stability and/or the amount of error in the zero-crossing detection. Errors in zero-cross detections may further exacerbate the phase delay problem through the filter, which in turn may further increase the errors in subsequent zero-crossing detections. This positive feedback effect may lead to system instability, in the form of a runaway condition, for example.